Geology of the Shenandoah National Park Region - Csmres Jmu ...

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topographic lineament, which they interpret as a north-northwest-striking fault (Harriston fault) near Grottoes and suggested this structure could have experienced Quaternary slip. Further research is required to evaluate the possibility of neotectonic activity along transverse faults. Many transverse faults are zones of structural weakness and strongly influenced the topographic character of the Blue Ridge Mountains; Simmons, Powell, Smith Roach, and Swift Run gaps are all located on transverse faults. Tectonic History Rocks and structures exposed in Shenandoah National Park formed in response to tectonic processes associated with the opening of ocean basins and the collision of continents throughout the past 1.2 billion years. Major tectonic events include the Mesoproterozoic Grenvillian orogen, Neoproterozoic Iapetan rifting, multiple episodes of Paleozoic collision, and Mesozoic crustal extension (Fig. 6). Mesoproterozoic granitoids and gneisses formed in the middle and lower crust along the margin of Laurentia during the long-lived Grenvillian orogeny that culminated in the amalgamation of the Rodinian supercontinent by 1,000 Ma. Older basement granitoids were emplaced during a magmatic interval 30 to 40 My in duration (1,180 – 1,140 Ma). A high-grade deformation event transformed these rocs into granitoid gneisses prior to the intrusion of the youngest plutonic suite by 1,080 Ma. The older deformation event is characterized by many northwest to east-west trending structures, but the kinematics of this event remain poorly understood. Lead isotopes in central and southern Appalachian basement massifs are distinctly different from other Laurentian Grenvillian provinces implying that Blue Ridge basement is not “native crust”, but rather was accreted to Laurentia during this long-lived orogeny. The small exposures of post-1,000 Ma paragneiss require uplift and erosion of the Blue Ridge basement followed by burial, metamorphism, and exhumation before Rodinia broke apart. From 730 to 700 Ma, plutonic and metavolcanic rocks of the Robertson River Igneous Suite were emplaced during an episode of crustal extensional. Although this tectonic event did not result in the formation of an ocean basin, it created significant accommodation space into which the thick sediments of the Fauquier, Lynchburg, and Mechum River sequence were deposited. The evidence of this first episode of Neoproterozoic crustal extension and rifting is preserved primarily to the east of Shenandoah National Park. Deposition of Swift Run sediments, emplacement of mafic dikes, and extrusion of Catoctin volcanics formed during a rift event that began ~570 Ma. Immediately southeast of the Park (in Greene and Albemarle counties), syn-sedimentary normal faults are associated with outliers of Swift Run and Catoctin formations. The regionally extensive Catoctin Formation may have been generated from plume-related processes that drove rifting along this segment of the Laurentian margin. The non-marine (Weverton) to nearshore-marine (Harpers and Antietam) rocks of the Chilhowee Group record the rift-to-drift transition as a passive continental margin developed along the Laurentian margin of the Iapetus Ocean in the Early Cambrian. Rocks in the Blue Ridge were progressively buried under a sequence of dominantly marine Cambrian and Ordovician sedimentary rocks (>8 km thick) deposited on the Laurentia continental margin. By the middle to late Ordovician, the Taconic orogeny had began in the Appalachians. Although numerous workers have argued for significant Taconic deformation in the Virginia Blue Ridge, recent geochronological studies provide little evidence of Ordovician activity. A late Ordovician to Silurian clastic wedge was deposited to the northwest of the modern Blue Ridge and significant subsidence continued into the middle Paleozoic (as preserved in Valley & Ridge sedimentary sequences). New Ar-Ar geochronology indicates that pervasive deformation in the basement and cover sequence occurred between ~360 and 310 Ma (late Devonian to Pennsylvanian). Deformation and metamorphism of this age falls between the classic Appalachian Devonian Acadian and Pennsylvanian-Permian Alleghanian orogenies. Zircon fission track data reveals the Blue Ridge had cooled through ~235˚ C by 300 Ma. The frontal Blue Ridge fault system slipped during the late Alleghanian (300 to 280 Ma) when the Blue Ridge massif was relatively cool. Evidence from the southern and central Appalachians suggests that middle to late Paleozoic tectonism was protracted and involved dextral 10
PALEOZOIC MESOZOIC NEOPROTEROZOIC MESOPROT. AGE (Ma) 180 200 220 240 260 280 300 320 340 360 380 400 420 440 460 480 500 520 540 560 580 600 change in scale 650 700 750 800 change in scale 900 1000 1100 1200 Stenian Tonian Cryogenian Ediacaran Cambrian Ordovician Silurian Devonian Carboniferous Permian Triassic Jurassic ?? ?? continued erosion Mafic magmatism Deposition of Culpeper Grp. exhumation and erosion brittle thrusting and emplacement of Blue Ridge sheet on Great Valley sheet contractional deformation, greenschist facies metamorphism high-strain zone movement, and foliation development in basement & cover sequence continued burial of Blue Ridge Continued Deposition in Valley and Ridge uplift, exhumation and erosion ?? continued burial of Blue Ridge Deposition of carbonates Deposition of Chilhowee Grp. Extrusion of Catoctin basalts Deposition of Swift Run Fm. exhumation and erosion Deposition of Mechum River Fm., Lynchburg Grp./Fauquier Fm. exhumation and erosion Intrusion of the Robertson River granitoids exhumation and erosion granulite facies metamorphism Intrusion of younger plutonic suite deformation and granulite facies metamorphism Intrusion of older plutonic suite 11 brittle normal faulting brittle normal faulting brittle/ ductile normal faulting ?? Atlantic Rifting Alleghanian Orogeny Acadian Orogeny Taconian Orogeny Iapetan Rifting Crustal Extension Grenvillian Orogenesis Figure 6. Generalized geologic and tectonic history of the Blue Ridge (from Bailey and others, 2006).
Page 1 and 2: 39 th Annual Virginia Geological Fi
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Geology of the Shenandoah National Park Region - Csmres Jmu ...

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